Linearity and control of nonlinear distortion may be used as criteria to judge the performance of amplifiers, including multi-channel power amplifiers (MCPAs). Amplifiers that have poorly controlled nonlinear distortion, including AM/PM (phase distortion) and AM/AM (amplitude distortion), may degrade signals on adjacent channels and deliver reduced signal quality on their own channels. Such amplifiers can also degrade the modulation quality on the signal, increasing the signal EVM (error vector magnitude).
Linearizing an amplifier may refer to adjusting its operation such that the output signal matches the input signal times a constant. Thus, if the amplifier does not amplify linearly, selected amounts of distortion may be added to either the input and/or output to correct for the nonlinearity. Amplifiers can use feedforward and predistortion techniques to improve linearity. Predistortion may be more popular because feedforward cancellation can be inefficient, at times becoming more difficult to implement when bandwidth increases. However, predistortion solutions sometimes involve increased complexity and larger die area, and/or they may improve AM/PM at the expense of AM/AM performance.